Conventional methods for separating suspended matters from a bulk (or bulk suspension) containing the suspended matters by using membranes include reverse osmometry utilizing pressure as a driving force, ultrafiltration method, micro-filtration method, electrodialysis utilizing a potential difference as a driving force and diffusion dialysis utilizing a temperature difference as a driving force. According to these methods, a continuous operation can be conducted, and separation, purification or concentration can be made without causing greatly a change in temperature or pH conditions during the course of separation. These methods enable various materials such as particles, molecules and ions to be separated. Further, since the operation capacity of small-size plants is large according to these methods, the operation can be efficiently carried out, energy required for the separating operation is small and low-concentration bulk suspensions which can be difficultly treated by other separation methods can be treated, and hence the above-mentioned methods are widely used. Membranes which are used in these separation methods include high-molecular membranes mainly composed of an organic high molecular material such as cellulose acetate, cellulose nitrate, regenerated cellulose, polysulfone, polyacrylonitrile, polyamide or polyimide, and porous ceramic membranes which are excellent in durability such as heat resistance and chemical resistance. When colloidal solutions are to be filtered, ultrafiltration membranes are used, and when fine particles are to be filtered, there are used micro-filter membranes having micropores suitable for use in the micro-filtration of fine particles.
With the advance of biotechnology, higher purity performance and higher precision have been demanded as mentioned above, and these systems have replaced conventional centrifugal separation and filtration using diatomaceous earth. The range of fields to which micro-filtration or ultrafiltration technique is applied is increasing because a continuous operation can be made and bulk treatment can be carried out; the filtration can dispense with the addition of a filter aid or a flocculant; separation efficiency is independent of a difference in specific gravity between cell and a suspension, and a clear filtrate can be obtained irrespective of the physical properties of the cultured liquor and the types of cells; high-concentration culture can be carried out and production efficiency can be increased; culture can be carried out a completely closed system without the leakage of cells; cells can be washed after concentration; and scaling up can be easily made and economic advantage are high. Though the filter membranes have many advantages, there are problems that a cake layer is formed by the influence of a concentration polarization to thereby produce a resistance to the flow of the transmitted fluid, and the resistance is increased by the clogging of the filter membrane to thereby cause rapidly and greatly a lowering in filtrate flux through the membrane when fine particles are separated by means of micro-filtration or ultrafilters. This is the greatest cause which obstructs the practical use of micro-filtration or ultrafiltration. Further, membranes used for micro-filtration or ultrafiltration are liable to be contaminated, and hence it is necessary to take a measure to prevent contamination.
Filtration systems include dead-end type filtration system wherein all fluids (bulks) to be filtered are passed through a filter medium (e.g., filter cloth or membrane) and a cake layer to thereby separate fine particles contained in the fluids (bulks). In conventional dead-end type filtration system, a high filtrate flux can be obtained in a stage where a fluid is passed through a filter membrane to thereby capture suspended matters in the inside of the filter membrane and to separate them, but a cake layer is formed in a stage where the suspended matters are captured on the surface of the filter membrane, and the cake layer produces a high filtration resistance when a large amount of a bulk (or bulk suspension) is treated or the specific resistance of the cake layer is extremely high. When such dead-end type filtration is carried out, filtrate flux through the membrane becomes small. For this reason, a dead-end type filtration system with periodic back wash and a crossflow type filtration system has been planned.
In the fields of drain disposal and the filtration of man-made water and pool water, it is known that back wash is carried out to recover the transmissible flux of the clogged filters. However, a system composed of a combination of the dead-end type filtration with back wash is a method which is developed in the field of drain disposal wherein the resistivity of the cake layer is relatively low. Accordingly, when the system as combined is used, the system is ineffective in the filtration of fine particles having a high resistivity, for example, in the separation of cells from fermentation liquors which produce enzyme, beers, wines, Japanese sake, Japanese soys, antibiotics, amino acids and organic acids such as acetic acid and oxalic acid. Filtration methods using a filter aid such as diatomaceous earth or pearlite are most widely used at present to treat these liquors. These methods are a type of a system wherein a block of fine particles of a filter aid is built to form a micro-filtration layer and the filtration is carried out through the fine particle layer. The methods are superior filtration methods which are used for a long time, and the methods are very effective methods for filtering a large amount of a suspension so far. However, these methods have problems in that the work requires many hands and a large amount of industrial waste is formed.
In the crossflow type filtration system, a bulk to be filtered is allowed to flow in parallel with the membrane surface of the filter membrane, the fluid is transmitted through the filter membrane into the opposite side and the flow of the bulk and that of the transmitted fluid cross meet at right angles. Thus, this filtration system is called crossflow type filtration system. In the crossflow type filtration system, the cake layer formed on the surface of the membrane is stripped off by the flow of the bulk in parallel with the filter membrane. Accordingly, the filtrate flux through the membrane is high in comparison with the dead-end type filtration system, and a large amount of the bulk can be directly and continuously separated, purified and concentrated. However, the resistivity of the filtration of the suspended matter is extremely high. Namely, when an ultrafilter or a micro-filtration membrane having a high pure water-filtrate flux, that is, a large fractional molecular weight is used to remove cells or high-molecular materials from the cultured liquor or the fermentation liquor, the filtrate flux through the membrane is rapidly lowered and it is difficult to keep the filtrate flux through the membrane which is high at early stage of the commencement of filtration. Accordingly, when the total amount of the transmitted fluid in the crossflow type filtration system is compared with that of the transmitted fluid in the dead-end type filtration system, the effect obtained by the crossflow type filtration system is small and an economic filtrate flux can not be sufficiently obtained.
The crossflow type filtration system is theoretically a high-degree separation technique as mentioned above. However, the degree of the filtrate flux through the membrane which is the greatest problem in the crossflow type filtration system is only slightly high in comparison with conventional dead-end type filtration system, and there is a problem that a sufficient high filtrate flux through the membrane can not be obtained when the crossflow type filtration system is used as the micro-filtration system.
In the embodiments of conventional separation of the suspended matter from the fluids, there are problems that not only the filtrate flux through the membrane is lowered by the cake layer formed on the surface of the membrane or the clogging of the membrane with the passage of the filtration time, but also the activity of cells is lost by a shearing force in the recycling of the bulk even when the crossflow type filtration system is used in place of conventional centrifugal separation or filtration system using diatomaceous earth, for example, in the separation of cells from the fermentation liquors.
As a method for increasing the transmitted flux, there has been conventionally tried so-call back wash wherein a cake layer or a deposit layer accumulated on the surface of the membrane on the bulk side of the filter membrane is intermittently removed by intermittently stopping the inflow of the bulk into the filter membrane, closing a valve provided on the transmitted fluid side of the filter membrane, intermittently removing pressure vertically applied to the surface of the filter membrane or reducing said pressure, or allowing the fluid to flow from the transmitted fluid side to the bulk side by applying pressure from the transmitted fluid side of the filter membrane. When a resistivity to the filtration of the suspended matter is low, the suspended matter accumulated to the filter membrane can be easily desorbed by back wash. However, in the case of high molecular components or cells which have high resistivity to the filtration of the suspended matter and high adhesion to the filter membrane, there are problems that they can not be sufficiently removed from the filter membrane by back wash, and the filtrate flux through the membrane can not be sufficiently recovered. In addition, there are problems that when the suspended matter desorbed from the filter membrane is left behind in the filtration system in carrying out back wash, the concentration of the suspended matter in the bulk is gradually increased, and optionally the viscosity of the bulk is increased with the result that the filtrate flux through the membrane is gradually lowered and the filtrate flux can not be sufficiently recovered even when back wash is carried out.
As a method for preventing the activity of cells from being lowered, there is conventionally carried out a method wherein a shearing force is reduced by lowering a circulating flow rate in the case of the crossflow type filtration system. However, when the shearing force is reduced, the effect of the crossflow type filtration system is reduced. Hence, there is a problem that the filtrate flux through the membrane is lowered when the filtration is carried out so as not to allow the activity of the cells to be practically lowered. Further, there is a problem that when a pump having a low shearing force such as a diaphragm pump is used to reduce the breakage of the cells, the pulsation of the pump is large and the effect of the crossflow type filtration system is reduced.
The present invention is intended to solve the problems associated with the prior art as mentioned above.